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Bacterial Photography

One of the things I'll be doing in my project over the next few weeks is designing a synthetic microbiological system; inserting different bits of DNA into bacteria to make them do ... well ... whatever I want really. There are a huge range of things you can get bacteria to do, and over the next ten weeks (while I'm doing my project) I'm going to try and cover one a week, just to give an idea of the scope and applications of synthetic biology, especially within bacteria.

So...for this week, bacterial photography: The picture above (image courtesy of UT/UCSF) is a coliroid, a picture taken by shining light onto a bacterial plate. This is done by putting genes that produce a black pigment under the control of a light-sensing bacteria. Bacteria in the light do not produce pigment, those in the dark do, creating a photographic image when light is shone on to a lawn of bacteria.

A more in-depth explanation of what is happening is shown in the diagram on the right (taken from this paper). The green blobs are photoreceptors; they sense light and in response they activate an intracellular protein portrayed as an orange blob (the double dotted lines are the bacterial cell membrane). This activated protein can then diffuse over to the DNA, and activate the promoter for a black pigment, which is then secreted out of the cell.

When light is present it blocks this activation process by stopping the action of the photoreceptors. Bacteria in the light will therefore not produce pigment, while those in the dark do produce pigment, leading to a darker colour on the agar plate.

The main challenge involved in this process was creating the photoreceptor. E. coli (the bacterial species used for this procedure) do not have any proteins for sensing light. Instead, a light sensing protein from a cyanobacterium was used, and held in place by fusing it to a trans-membrane protein (in the diagram above the cyanobacteriumreceptor is the green blob while the E. coli trans-membrane protein is the dark black line). This creates a chimeric protein, which can be put on a small circle of DNA (known as a plasmid) and inserted into the bacteria, along with the genes for the pigment. The bacteria can then be grown on a large plate, ready to be used for photography.